(1) Technical Field
The invention relates to methods for removing protuberances at the surface of a semiconductor wafer and more particularly to a method for removing the silicon dioxide protuberances (such as the "bird's heads") according to a chem-mech polishing technique. The "bird's heads" are produced by conventional Recessed Oxide Isolation (ROI) processes.
(2) Description of the Prior Art
In the monolithic integrated circuit technology, it is usually necessary to isolate various active and passive devices from one another in the integrated circuit structure. Among different possibilities, complete dielectric isolation is widely used because of the significant advantages this technique offers, including the reduction of parasitic capacitances, improved performances, also, the ability of allowing the abutting of certain regions (such as the base or the emitter) against the isolation sidewalls, thereby finally resulting both in greater integration densities and in self-alignment capabilities.
One form of dielectric isolation involves the formation of recesses or grooves at isolation regions in the silicon substrate covered with a thermally grown SiO.sub.2 and Si.sub.3 N.sub.4 layer. During the formation of the grooves, the remainder of the silicon substrate is coated by a protective photoresist film. During CF4 RIE etching the Si.sub.3 N.sub.4, SiO.sub.2 and the silicon are etched at isolation sites which are not covered by photoresist. Following the formation of the grooves by conventional etching, the silicon substrate is submitted to a conventional thermal oxidation step, whereby the exposed silicon in the grooves is converted into silicon dioxide and fills up the grooves as well as oxidizing further into the silicon, to form the isolation regions.
Unfortunately, the use of this isolation technique, so-called ROI, results in two major drawbacks known in the literature, as the "bird's beak" and the "bird's head" problems.
"Bird's heads"0 and "bird's beaks" are respectively non-planar silicon dioxide at the top periphery of the groove and the lateral oxidation underneath the silicon nitride layer. In particular the "bird's head" is a ridge-shaped protuberance which delineates the periphery of the recessed oxide isolation regions formed by the ROI process. The process leaves the silicon substrate fully coated with either a thick (at the ROI locations) or a thin (at other locations) SiO.sub.2 layer. The "bird's head" results from lateral, vertical and stress enhanced oxygen diffusion during the oxidation step. More details concerning the stress phenomenon and the different troubles caused by the ROI process, can be found in the published European patent application No. 82106651.1 (Publication Number 071203) to Kemlage, and assigned to the same assignee as the present invention.
First of all, where associated with the "bird's head", the "bird's beak" produces difficulties when it is necessary to subsequently achieve good abutted diffusion regions against the sidewalls of the dielectric isolation region.
Secondly, the growth of the "bird's head", of an approximate height of 0.5 .mu.m, produces a bumpy or irregular silicon surface topography during the ROI process. As a first consequence, the non-planarity of the silicon surface can lead to the formation of polysilicon rails during the timed Reactive Ion Etching (RIE) of doped polysilicon used to provide electrical contacting to the base and/or emitter regions. These polysilicon rails contribute to electrical shorts between adjacent devices. This phenomenon can be understood from the teachings of an article entitled "Preventing formation of polysilicon rails" by C.G. Jambotkar, and published in the IBM Technical Disclosure Bulletin. Vol. 25. No. 12, May 1983, pp 6607-6608. Another consequence of the non planarity of the substrate surface, is the potential breakage of metal lands to be subsequently formed. Finally, it is widely recognized that a planar surface is beneficial to multi-level metal personalization schemes, by increasing significantly both wiring densities and reliability of the final devices.
Recently, the isolation has been considerably improved by forming a trench in the center of the ROI region. The trench may be filled with doped polysilicon to provide an electrical contact with the substrate where desired, or in other applications, the trenches are filled with a dielectric insulating material, such as intrinsic polysilicon or silicon dioxide to provide additional isolation between devices.
There have been many attempts so far to solve the acute problems mentioned above resulting from the formation of these SiO.sub.2 ridge-shaped protuberances.
According to several references, different manufacturing processes have been suggested to define new processing steps in order to avoid such a formation. Illustrating this approach, the above mentioned European patent application describes a process where the underlying SiO.sub.2 layer is replaced by a silicon oxynitride (SiO.sub.x N.sub.y) layer. Also in U.S. Pat. No. 3,886,000 to R. L. Bratter, the underlying SiO.sub.2 layer is eliminated and a silicon oxynitride layer is formed directly on the silicon substrate. All these techniques have been wanting in demonstrating effectiveness and controllability. In addition, they employ materials such as SiO.sub.x N.sub.y which are not widely used in this industry.
Unlike this approach, other proposals do not remove the problem but find a partial solution of the problem after it occurs. For example, U.S. Pat. Nos. 4,025,411 to Y. Hom-Ma and 4,039,359 to N. Nakamoto describe the problems caused by the "bird's heads" and suggest to solve it by an etching of the ridge, in order to planarize the structure.
In the former reference, the protuberances or "bird's heads" projecting above the surface of the substrate, which directly result from the ROI process, are removed by: first planarizing the structure with a planarizing medium such as a photoresist (having approximately the same etching rate as SiO.sub.2), then etching back both SiO.sub.2 and the photoresist with the same etching rate, for instance in a RF sputtering equipment. The result is a silicon wafer with a flat surface. In the latter reference, an alcohol solution of a doping impurity such as boron, is applied to the substrate surface having "bird's heads". By a thermal treatment, the solution is converted into a boron doped SiO.sub.2 film, which has approximately the same etching characteristics that the underlying SiO.sub.2. The substrate is immersed in an appropriate etchant, which etches both oxides at the same rate. According to this process, the "bird's heads" are smoothed, however the process claims only a reduction of the step height (e.g. from 0.6 to 0.2 microns), but not a complete removal. In addition, with this process, P doped regions are formed in the silicon locations covered with said boron doped SiO.sub.2 film and therefore may constitute an additional limitation to this particular process.